The role of opioid kappa1 and kappa2 receptors in reperfusion cardiac injury was studied. Male Wistar rats were subjected to a 45-min coronary artery occlusion followed by a 120-min reperfusion. Opioid kappa receptor agonists were administered intravenously 5 min before the onset of reperfusion, while opioid receptor antagonists were given 10 min before reperfusion. The average value of the infarct size/area at risk (IS/AAR) ratio was 43 - 48% in untreated rats. Administration of the opioid kappa1 receptor agonist (-)-U-50,488 (1 mg/kg) limited the IS/AAR ratio by 42%. Administration of the opioid kappa receptor agonist ICI 199,441 (0.1 mg/kg) limited the IS/AAR ratio by 41%. The non-selective opioid kappa receptor agonist (+)-U-50,488 (1 mg/kg) with low affinity for opioid kappa receptor, the peripherally acting opioid kappa2 receptor agonist ICI 204,448 (4 mg/kg) and the selective opioid ?2 receptor agonist GR89696 (0.1 mg/kg) had no effect on the IS/AAR ratio. Pretreatment with naltrexone, the peripherally acting opioid receptor antagonist naloxone methiodide, or the selective opioid kappa2 receptor antagonist nor-binaltorphimine completely abolished the infarct-reducing effect of (-)-U-50,488 and ICI 199,441. Pretreatment with the selective opioid ? receptor antagonist TIPP[psi] and the selective opioid µ receptor antagonist CTAP did not alter the infarct reducing effect of (-)-U-50,488 and ICI 199,441. Our study is the first to demonstrate the following: (a) the activation of opioid kappa2 receptor has no effect on cardiac tolerance to reperfusion; (b) peripheral opioid kappa1 receptor stimulation prevents reperfusion cardiac injury; (c) ICI 199,441 administration resulted in an infarct-reducing effect at reperfusion; (e) bradycardia induced by opioid kappa receptor antagonists is not dependent on the occupancy of opioid kappa receptor.